Please refer to FIG. 1, which is a sectional diagram of a liquid-cooling system in prior art. Details of FIG. 1 could be obtained by referring to U.S. Pat. No. 8,240,362 and U.S. Pat. No. 8,245,764. A surface of the IC chip 1 is contacted with a surface of the heat sink 10 so that the thermal energy generated by the IC chip 1 can be conducted to the liquid tank 11 via the heat sink 10. An impeller 12 driven by electric motor (not shown) is integrated in the liquid tank 11 for exhausting the high-temperature liquid in the liquid tank from the exit 16 and sending back the low-temperature liquid, which is cooled down by heat exchange in a heat radiator (not shown), to the liquid tank 11 via the inlet 15. However, in the cooling system mentioned above, the high-temperature liquid and the low-temperature liquid might be usually mixed in the liquid tank 11 so that an efficiency of heat dissipating is reduced and the requirement of heat dissipating for those IC chips with increasingly working temperature cannot be met.